Anion transport blocking agents were found to interfere with the clotting of fibrinogen by thrombin and with the attachment of thrombin to platelets. Platelet metabolic responses to thrombin were inhibited proportionally to the degree of inhibition of thrombin attachment, suggesting that platelet fibrinogen is the most likely receptor for thrombin and that the action of anion transport blockers on platelets does not support the "chemiosmotic" mechanism of cellular secretion.